1. Technical Field
The present invention relates generally to photoreceptors designed to operate at low light levels and more particularly to circuits for increasing the reliability of photoreceiver information acquired under conditions in which a signal-to-noise ratio is relatively small.
2. Related Art
An accurate determination of the path of a device across a surface is important in a variety of applications. For example, if a faithful representation of an image of a scanned original is to be acquired, there must be accurate information as to the travel of the scanning device along the original. Typically, the captured image provided by a scanner is a pixel data array that is stored in memory in a digital format. A distortion-free image requires a faithful mapping of the original image to the pixel data array.
U.S. Pat. No. 5,149,980 to Ertel et al., which is assigned to the assignee of the present invention, describes use of a cross-correlation function to determine the relative movement between an original and an array of photoelements in a given direction. The patent notes that the one-dimensional approach can be extended to determine the vector of two-dimensional relative movement between the original and the array, so as to track translation, rotation and scaling in a two-dimensional plane.
The patent to Ertel et al. describes use of an optical sensor array to collect a "signature" of an original. The signature may be provided by illuminating and imaging the surface texture or other optical characteristics of the original. The light intensity will vary on a pixel-by-pixel basis with variations in surface texture. By cross-correlating images of the surface of the original, relative movement between the array and the original can be ascertained.
A critical element of the design of a system such as the one described by Ertel et al. is circuitry to maintain the signal-to-noise ratio of each photoelement sufficiently high to reliably determine the signature of the original. If the signal is the difference in reflectivity from pixel to pixel as a result of slight variations in paper texture of a white paper, the variation in reflectivity may only be approximately six percent. The overall resolution goals translate into a relatively low signal-to-noise ratio for each photoelement, with the desired signal being the small change in reflectivity of the medium of interest and the dominant noise term being shot noise of the photodiode, which is amplified by the phototransister action, as a result of the fixed portion of the reflectivity.
Co-pending U.S. patent application Ser. No. 08/591,848, filed on Jan. 25, 1996, to Baumgartner et al., which is assigned to the assignee of the present invention, also describes the use of circuitry for an optical sensor array to collect a "signature" of an original. Applicants hereby incorporate U.S. patent application Ser. No. 08/591,848 by reference in its entirety. In the preferred embodiment, the sensor array includes a pair of navigation sensors disposed at opposite ends of an imaging sensor for tracking movement of the scanning device relative to the original.
In the reference to Baumgartner et al, each navigation sensor is a two-dimensional array of photoelements that is formed on an integrated circuit substrate that includes readout and signal processing circuitry. The position accuracy necessary over the range of a pixel distance of 40 .mu.m is 2.0 .mu.m. The very high positional accuracy requires individual photoelements that are no larger than tens of microns in length in order to acquire sufficiently differing signals from element to element. For a desired pixel size on a paper original of 40 microns, a magnification of 1.5 is achieved by imaging optics comprising microlenses so that photoreceptor elements having a pitch 60 .mu.m.times.60 .mu.m are suitable. Each navigation sensor is comprised of an array having sixty-four columns and thirty-two rows, wherein each row is comprised of sixteen photoelement pairs. Each photoelement pair comprises a single photocell. Each photoelement includes essentially two portions. The first portion includes the phototransistor for receiving light, and the second portion includes a servo amplifier circuitry for biasing the phototransistor base and an integration capacitor for generating a charge from photo-generated current. The integrated charge on the capacitor is periodically read and processed for determining the amount of light falling on the photoelement during a given reading period. An area ratio of to the second portion is commonly referred to the fill factor. Unity as a fill factor for a particular photosensitive microchip ideally represents the most efficient use of microchip space since all light shining on the photoelement would then be converted to photocurrent. The circuit layout of Baumgartner et al achieves a fill factor of approximately 0.40 by utilizing a relatively small number of components in the photoelement.
However, although meritorious to an extent, the photoelement layout of Baumgartner et al. includes several disadvantages. First, the photocell construction of Baumgartner et al is asymmetrical because the adjacent pairs of photoelements represent mirror images of each other. More specifically, the servo amplifier circuitry and integration capacitors for a photoelement pair are oriented adjacent to one another with the phototransistors occupying the remainder of photoelement space. That asymmetrical layout exacerbates image blurring, which occurs when an image on paper moves relative to the photoreceptor array during image acquisition. Additionally, the asymmetrical arrangement is an inefficient use of space on an integrated circuit substrate, which limits the fill factor. The relatively low fill factor requires the use of micro-lenses per pixel to increase the effective fill factor, which increases the cost and complexity a scanning device utilizing the photosensitive array.
What is needed is circuitry which permits reliable use of a photoelement circuitry by providing a photoelement layout which minimizes image blurring during image capture while maximizing the fill factor to achieve navigational chip light use efficiency, which is especially important in low light level applications.
Other objects, features and advantages of the present invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings.